What is the source of your quoted text?
Isolation is a different mechanism than filtering. Re-clocking is also different mechanism than filtering.
From Uptone’s website:
“The heart of what makes our switch so unique is use of ACTIVE, HIGH-SPEED, LOW-JITTER DIFFERENTIAL DIGITAL ISOLATOR chips in conjunction with ULTRA-LOW JITTER DIFFERENTIAL RE-CLOCKING FLIP-FLOPS. No other Ethernet switch on the market does this. Implementing this architecture correctly is difficult and costly!”
You wrote: “Instead, one should deal with the noise upstream as much as possible”. Exactly!! That’s why the ER makes perfect sense. It provides isolation and reclocking. Isolation helps to ensure leakage current doesn’t enter the Antipodes server. And reclocking helps to reverse the harm that cheap clocks in consumer gear do to the signal.
Yes, they are different but apparently in ER filtering is side by side with other functions if there are any.
Regarding the claim, Audio Science Review found absolutely no difference between $20 switch and this $640 ER one. Audio Precision instrumentation was used, so no mistake related to an unreliable instrument.
Re dealing with the noise upstream, yes one should use something like Antipodes CX (until Audio Science Review debunks it, of course, just like it debunked ER, which consequently should not be used - I trust Audio Precision measurements).
It is logical that ER must, by default, deal with noise. Why? Because they state jitter reduction as one of their goals, as you pointed out. It is widely known that 3 things affect jitter:
noise
bandwidth
clock
If a device, any device, claims jitter reduction it must deal with all three (usually, they deal with noise and clock only; Antipodes was the first that added bandwidth to this mix).
Now, if we focus on ER noise reduction (they do not talk about bandwidth), Audio Precision measurements measured no reduction at the output terminals of the DAC... Except if Audio Science Review is in the business of lying to the public (if they lie, I am certain they won't last; I think it is highly unlikely they are lying. I am not saying it is impossible, just that it is highly unlikely).